1. Field of the Invention
The present invention relates to a friction material, and which may be used to such as vehicles to reduce braking noises. More specifically, the present invention relates to a brake pad or a brake lining which may be used to braking a vehicle.
2. Description of the Related Art
A friction material to be used to a brake pad, a brake lining or a clutch facing of vehicles, for example, automobiles, is generally formed as a composite material composed of reinforcing fibers, a friction modifier, a binder, etc.
However, in the vehicle where the friction material has been used to the brake pad, there are some problems. When passing puddles or washing the vehicle, a rotor and the brake pad are wetted by a water. Otherwise, when parking outdoors in night for a long time, the friction material has absorbed a moisture in pores thereof. In such cases, the brake is dragged by a creeping force when starting at low speed and a low frequency noise tend to be generated while the water or the moisture are dried and when a vehicle (for example, an automatic (AT) car) starts, that is, the brake pedal is pressed and the change gear is brought into a running range for loosening until the brake is completely released.
Further, when temperature of the brake is increased by repeating the braking operation and the component contained in the friction material is adhered to the rotor, odd noises of low frequency are created at starting the vehicle also due to stick-slip which might be caused between a film of the component and the friction material.
This odd noise of low frequency issued when starting the vehicle is specifically called as a xe2x80x9ccreep noisexe2x80x9d.
The creep noise produced at starting the vehicle is the odd noise of low frequency of about 100 to 400 Hz which is produced due to instantaneous variation of a friction coefficient between the friction material and a counterpart material at ultra low speed. The creep noise gives unpleasant feelings to users in general markets, and recently has been demanded to take any measures.
It is accordingly an object of the invention to provide a friction material for effectively reducing the creep noise which is produced when starting the vehicle.
Inventors made many studies on kinds of friction modifiers for providing friction materials which may effectively reduce the creep noise generated at starting the vehicle.
As the friction modifier for preventing the creep noise, zeolite, antimony oxide and fluorine based polymer of specified amounts were selected, and noticing an effective prevention of the creep noise by the combination of three components, thus the invention has been attained.
Namely, the invention has solved the problem by a friction material including reinforcing fibers, a friction modifier, a lubricant, and a binder, characterized by containing, as a part of the component of the friction modifier, zeolite in an amount of from 2 to 5 wt %, antimony oxide in an amount of from 0.8 to 2 wt % and fluorine based polymer in an amount of from 0.2 to 1 wt %.
For producing the friction material of the invention, the friction material composed of the reinforcing fibers, the friction modifier, the lubricant, and the binder is mixed with the above mentioned raw materials, previously molded in accordance with ordinary production methods, and thermally formed.
As to the friction material of the invention, the reinforcing fibers may include a heat resistant organic fiber, for example, an aromatic polyamid fiber or a frame resistant acrylic fiber. Above of all, an aramid fiber or an aramid pulp may be preferably used because of high strength and high heat resistance. The preferred aramid fiber and aramid pulp may include Kevlar, a trade name (made by Du Pont) or Twaron, a trade name (made by Twaron Products).
For the binder, there may be enumerated, for example, thermosetting resin such as a phenolic resin (including straight phenolic resin and various phenolic resins modified by rubber), or the like and a polyimid resin.
Further, as the friction modifier for adjusting the friction property of friction coefficient, enumerated are, for example, inorganic friction modifiers such as alumina or silica, magnesia, zirconia, chromium oxide, potassium titanate or quartz, organic friction modifiers such as synthetic rubber, or cashew resin, and as the lubricant, there may be enumerated, for example, graphite or molybdenum disulfide.
According to the invention, the friction modifier for preventing generation of the creep noise is characterized by containing zeolite, antimony oxide, and fluorine based polymer.
For the fluorine based polymer, there may be enumerated ethylene polymer tetrafluoride, tetrafluoroethylene polyethylene hexafluoropropylene copolymer, polyviniliden fluoride and the combination of these elements. Among these elements, the ethylene polymer tetrafluoride is preferred.
As to the mixing amounts of the friction modifier to be used to the friction material of the invention, it is required that zeolite is 2 to 5 wt %, antimony oxide is 0.8 to 2 wt % and fluorine based polymer is 0.2 to 1 wt %.
In case the mixing rate of each of the friction modifiers is less than the lower limit of the specified values, the creep noise will not be effectively eliminated.
On the other hand, if the amount of zeolite is higher than 5 wt %, a wear-resistance is reduced and a commercial value is lowered accordingly. Further, if the amount of antimony oxide is higher than 2 wt %, a fade-resistance is lowered. Further, if the amount of fluorine based polymer ia higher than 1 wt %, the braking effect is decreased.
The brake pad is produced in that a sheet is pressed into a predetermined shape, subjected to a degreasing treatment and a primer treatment, and a pressure plate coated with an adhesive and a pre-formed body are formed at a predetermined temperature and at a predetermined pressure in a thermally forming procedure into an integral combination of both members, and this combination passes through an after cure and a finally finishing treatment, said pre-formed body being formed by mixing the reinforcing fibers of heat resistant organic fiber, the lubricant, the friction modifier and the binder of heat setting resin, and forming (preforming) the friction modifier completely homogenized by stirring, at room temperature and at a predetermined pressure. The above mentioned steps are the same as those of the prior art.
The invention will be specifically described in reference to an Example. It should, however, be noted that the invention is not limited thereto.